The economic scale up of electrochemical reactors and processes has been challenging due to the low economy of scale factor inherent to such devices. This challenge can be encountered by electrochemical processes including electrowinning, electrorefining, electroplating, electrocoagulation, hypochlorite and disinfectant generation, hydrogen generation, and fuel cell and battery operation. A variety of factors can play a role in the inability to economically build and operate scaled up electrochemical reactors and processes.
Generally speaking, productivity in electrochemical reactors and processes is directly proportional to the electrode area and current density at a given efficiency. Accordingly, scale up tends to require larger electrodes, an increased number of electrodes, and/or increased power demands, all of which raise the cost of operating the electrochemical reactor and process.
Productivity can also be a challenge when the electrochemical process is limited in rate due to the kinetics of the desired transformative processes and mass transport. Additional challenges are encountered when secondary processes are competing with the desired transformation. Particularly challenging are electrochemical processes involving several steps along the reaction pathway resulting in the slow transformation of reactants to products and lower productivity. Such steps can include dissolution, diffusion, electrode or catalyst surface adsorption/desorption, electron transfer, ion transfer, molecular rearrangement, breaking or making of chemical bonds, and chemical reactions of intermediates. Further complicating matters is that the desired process may occur at a similar electrode potential (voltage) as other undesired processes, such as side reactions that consume electricity, may consume the desired product or produce a poor product. The aforementioned issue of poor process selectivity can also lead to reduced productivity.
A variety of methods are employed to compensate for slow kinetics, limited mass transport and reduced selectivity of certain electrochemical processes. Some of the most common methods include the use of high surface area electrodes, process-selective electrocatalysts, pH adjustment, temperature control, chemical additives, elevated reactant concentration, mixing and turbulence, separation of electrode processes, potential control, and minimizing shunt losses or shorting (both electrical and chemical). The above list is only exemplary as there are many useful methods demonstrated in the literature. The aforementioned methods are effective for optimizing single phase electrochemical reaction processes in general, but are not as effective or practical for optimizing multiphase electrochemical processes.
Electrochemical processes that involve the interaction of multiple phases at an electrode surface, such as gas and liquid, add another level of challenge when designing highly productive electrochemical devices. Achieving uniform distribution of gas and liquid components over large-scale electrodes is challenging from a practical design and operations perspective. Often, the multiphase electrochemical process involves the transformation of gas-phase materials (such as the reduction of oxygen or carbon dioxide), which must be present at the electrode surface simultaneously with the liquid (electrolyte) phase for a transformation to occur. Examples of reactors designed to afford high surface area access to gaseous and liquid phases include those employing gas diffusion electrodes, packed beds of conductive particles, dimensionally stable porous conductive media, rotating electrodes, vortex-flow or high sheer flow electrodes. Many of the above multi-phase reactor approaches are feasible in the laboratory, but few are practical for economic scale up, production, and operation.
Particularly desirable applications of electrochemical reactors are on-site, mobile, and distributed systems for generating cleansers and disinfectants, decontamination and remediation, water treatment and recycle, waste treatment, and chemical production. The design of a cost-effective multi-phase electrochemical reactor is critical to making several potential applications commercially viable. Although electrochemical systems are not cost competitive to many bulk commodity chemical manufacturing processes at the largest scales, they do offer cost advantages for medium to small distributed applications, remote operations, and rapid deployment systems while helping to reduce transportation costs and provide safety advantages for end-users, facilities, and populations around transportation corridors.
Electrochemical reactors should ideally be compact, convenient and safe to operate in a variety of settings without extensive supporting utilities or infrastructure. Examples of application settings include, but are not limited to, laundry machines, clean-in-place equipment, desalination membrane cleaning, food service cleansing applications, medical facilities, dairy and farming operations, trailer-mounted environmental remediation and decontamination systems, remote site chemical generation and waste treatment (mining, offshore oil rigs, marine vessels), and on-site management of highly toxic and radiation-contaminated waste.
The use of multi-phase electrochemical reactors is desirable for enabling cathodic processes, with known examples including hydrogen peroxide production, cleanser generation, co-generation of products, nitrate destruction, electrolytic water treatment, dechlorination, deozonation, and carbon dioxide reduction to fuels or chemicals. Potentially valuable cathodic processes involve the reduction of reactants such as oxygen or oxides (e.g., nitrate, nitrite, nitrous oxides, carbon dioxide) to various products. Reduction of such materials is a kinetically slow process resulting in low specific current densities (less than approximately 1000 A/m2 specific electrode area) or poor current efficiency for the desired process. Likewise, concentrations of such materials are often low or diffusion limiting, thus further inhibiting reaction rates, specific current density, efficiency and overall productivity. The above issues result in capital costs that make such electrochemical processes cost prohibitive.
The use of such reactors can also be desirable for enabling anodic processes to be conducted without the generation of hydrogen gas as a process byproduct by means of an oxygen depolarized cathode. This capability is a useful for operations in confined spaces where the risk of flammable gas buildup is undesired, such as in occupied buildings, marine vessels, and underground mining operations. Examples of anodic processes include chlorine, hypochlorous acid and hypochlorite generation, ammonia oxidation, desulfurization and deodorization of gases, organic contaminant destruction, and electrocoagulation. The use of such reactors can also enable co-generation of several of the above products or combining processes depending on the combination of inputs.